Absorbent articles, such as disposable diapers, are generally provided with absorbent cores to receive and retain body fluids. In order for such absorbent articles to function efficiently, the absorbent cores must quickly acquire body liquids into the structure from the point of application and subsequently distribute the body liquids within and throughout the absorbent core to provide maximum absorbency and containment. In order to efficiently perform these functions, the absorbent core should substantially maintain its shape during manufacture, shipping and packing, and fitting and usage by the user, including usage subsequent to initial wetting by the user.
Various material have been employed as the absorbent media in disposable absorbent articles. The most commonly utilized absorbent media in recent years is chemically pulped wood fibers provided in a highly individualized, i.e., disintegrated, state referred to in the art as "fluff". Fluff is typically air-laid in the form of webs, which are then utilized as the absorbent core of absorbent disposable articles.
Performance of disposable diapers has, in recent years, been improved through the incorporation into the absorbent cores of polymeric gelling materials, which have the capability of absorbing fluids to form gels which lock-away the fluids and which do not release such fluids upon application of pressure to the absorbent core (e.g., by the user rolling about after discharging fluids into the absorbent article). Due, at least in part, to the performance capabilities of disposable absorbent articles having absorbent cores of chemical pulp fibers and polymeric gelling materials, the trend has been to manufacture disposable articles having absorbent cores which are thinner, and of higher density, than conventionally practiced prior to the widespread usage of polymeric gelling materials.
Although the chemically pulped fibers commonly used in disposable absorbent articles provide good overall performance, especially in terms of web integrity, it is desirable to utilize fibers in disposable absorbent articles which are more efficient from the standpoint of utilization of natural resources. Typically, the chemically pulped fibers that are utilized as fluff in absorbent articles constitute only about 40% of the wood source from which the fibers are processed. Additionally, it is desirable to provide absorbent cores of fibers made by processes which require reduced levels of pulping chemicals (e.g., sulfate in the "Kraft" chemical pulping process) relative to processes conventionally used to make chemical pulp fibers. One type of absorbent fibrous media which fits these objectives is known generally as mechanical pulp, which term includes such variants as thermomechanical pulp and chemithermomechanical pulp. The processes used to make these mechanical pulps, which are known to those skilled in the art, provide substantially higher yields (typically in excess of 85%) than chemical pulp processes. In addition, mechanical pulp processes involve reduced usage of processing chemicals.
In spite of the above advantages, mechanically pulped fibers have not been commonly utilized in commercially marketed disposable absorbent articles, except in countries where environmental concerns are especially prevalent. One of the drawbacks conventionally associated with mechanically pulped fibers is that they provide relatively low strength when formed into air-laid webs. The use of such low strength webs as absorbent cores in disposable absorbent articles has conventionally required the use of a strength-imparting envelope. Webs of low strength mechanical fibers including an interior reinforcing layer in addition to the required tissue envelope layers are also known. See, for example, U.S. Pat. No. 4,327,729, L. W. King, issued May 4, 1982. Unfortunately, the presence of an additional structural element between the absorbent core and the topsheet of an absorbent article can impede the rapid absorption of fluids discharged during use. Other drawbacks traditionally associated with low strength mechanical pulp webs are that mechanical pulp fibers are less hydrophilic than chemical pulp fibers and have reduced ability to retain fluids in the void spaces between fibers. Both of these factors contribute to reduced performance in the context of absorbent article performance relative to chemical pulp fibers.
In view of these drawbacks, it is desirable to provide a disposable absorbent diaper which incorporates, as its primary absorbent fibrous media, high yield, mechanical pulp fibers. It is an object of the invention to provide such a disposable absorbent article which has absorbency and containment performance, as well as absorbent core structural integrity, comparable to disposable absorbent diapers having chemically pulped fibers as the principal absorbent fibrous media. It is particularly an object of this invention to provide disposable absorbent diapers having thin absorbent cores made from mechanical pulp fibers which provide absorbency and containment performance, as well as absorbent core structural integrity, comparable to disposable absorbent articles chemical pulp fibers and polymeric gelling materials incorporated therein.
These objectives, and other benefits as may become apparent to those skilled in the art, are believed to be achieved by the invention which is described hereinafter.